The curtain control devices of the prior art utilize manual transmission modules having strings or bead chains for controlling the opening/closure of the curtains. As a more convenient control device, electric transmission module for the opening/closure of the curtains such as the one disclosed in FIG. 1, comprises a motor (not shown in the figure), an active wheel 61, button structures 62 and a box 63. A string (not shown in the figure) is wound around the lateral groove of the active wheel 61, and is pinched by a pair of passive wheels 64 on both sides of the active wheel 61. The motor drives the active wheel 61, which in turn drives the string for controlling opening/closure of the curtains. Restricted by the pinching force between the active wheel 61 and the passive wheels 64, sliding of the string with respect to the groove of the active wheel 61 often occurs, which hinders the electric transmission. Especially after using for an extended period of time, the surface of the string becomes so smooth that the transmission could be badly influenced.
Referring to FIG. 2, a motor is used to drive an active wheel 70. Passive wheels 72, 73, further guide a string 71 that passes around the active wheel 70, so that the two-way motor can control the opening/closure of the curtains. The prior art has the same disadvantages of the above first prior art.